1. Field of the Invention
The present invention relates to a superconducting coil, and more particularly, it relates to a superconducting coil for a superconducting magnet which is applied to a magnetic resonance diagnostic apparatus or the like and cooled by a cryogenic refrigerator, for example.
2. Description of the Background Art
In general, superconducting magnets are cooled by two types of methods including a method of dipping and cooling a superconducting magnet in a refrigerant such as liquid helium or liquid nitrogen, and a method of thermally connecting a superconducting magnet directly to a cold head of a cryogenic refrigerator.
In the latter superconducting magnet cooled by a cryogenic refrigerator, a superconducting conductor is generally wound on a coil former (bobbin) in the form of a pancake or solenoid. Japanese Patent Laying-Open No. 6-174349 (1994) proposes means of interposing a mixture of silicon grease and a powder material having excellent thermal conductivity in a connecting portion between the cryogenic refrigerator and the superconducting magnet while filling up clearances between coil wires and those between the coil and the bobbin with the mixture, in order to improve the cooling efficiency for the superconducting magnet having such a structure.
While the superconducting coil can be cooled to a prescribed very low temperature in a short time in the superconducting magnet proposed in the aforementioned gazette, however, remarkable heat generation is caused by an ac magnetic field or a shunt current to disadvantageously result in normal conducting transition of the superconducting coil when the superconducting coil is rapidly excited in the state cooled to the prescribed very low temperature. Such heat generation cannot be suppressed, and hence the superconducting magnet cannot be continuously driven and no stable operation can be attained.